Zeolites are crystalline, hydrated aluminosilicates of Group I and II elements. Structurally, zeolites comprise a framework based on an infinitely extending three-dimensional network of SiO.sub.4 and [AlO.sub.4 ].sup.-1 tetrahedra linked through common oxygen atoms. The framework structure encloses cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration. The isomorphic substitution of silicon by aluminum gives rise to a net negative charge compensated by cations. Zeolites can be represented by the formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
where M is the compensating cations with valence n, x is .gtoreq.2, and y=10-10,000.
Different zeolites may have different Si/Al ratios and the tetrahedral SiO.sub.4, [AlO.sub.4 ].sup.-1 can also be isostructurally substituted by other elements such as Ga, Ge, Mn, Ti, and P, generating a molecular sieve. In an extreme case, zeolite molecular sieves may have a Si/Al ratio of infinity. Zeolite molecular sieves do not have a net negative framework charge, exhibit a high degree of hydrophobicity, and have no ion exchange capacity. Silicalite-1 and silicalite-2 are examples of such zeolite molecular sieves.
Silicalite-1 is the end-member of zeolite Pentasil, also referred to as zeolite ZSM-5. Silicalite-1 has the same structure as ZSM-5, but has a Si/Al ratio of infinity, with a high hydrophobicity and has no ion exchange capacity.
Zeolites have been used commercially in two different ways. In one way, zeolites are in the form of granules. These exhibit high porosity with a pore size between 3-12 .ANG., depending on the type of zeolite. Zeolite particles have been used in adsorption separation processes and in shape-selective catalytic processes. The adsorptive separation process on granular molecular sieves is a non-continuous batch process involving alternate adsorption and desorption, and has low economic feasibility.
A second commercial use of zeolites has been with ceramic membranes. These exhibit high thermal, chemical and mechanical stability, and can be used in continuous separation processes. The economic advantage of using a membranous article having a zeolite surface which would allow carrying out such processes continuously have been recognized (U.S. Pat. No. 4,699,892). However, it has proved very difficult to produce a ceramic-zeolite membrane with a pore size of less than 10 .ANG. (Goldsmith (1988) J. Membrane Sci. 39:197).
Zeolites have had a limited application as membrane materials for continuous separation processes. Only recently have studies demonstrated the potential of zeolites as filling materials for improving the performance of polymeric membranes, as membranes for separation, and for catalytic membrane reactor applications.
For example, Hennepe et al. (U.S. Pat. No. 4,925,562) describes a zeolite-filled polymer membrane in which hydrophobic silicalite-1 particles were homogeneously dispersed in a polydimethylsiloxane (PDMS) polymer. When used in a pervaporation process to separate alcohols from water, the membrane was shown to have improved properties of selectivity and flux. The extent of the improvement was proportional to the silicalite content of the filled membrane.
A method for synthesizing a continuous crystalline molecular sieve membrane without any carrier matrix is described by Haag et al. (U.S. Pat. No. 5,019,263 and U.S. Pat. No. 5,069,794). In one example, a Teflon plate is immersed at 453 K. for nine days in a precursor gel consisting of a mixture of tetrapropylammonium bromide, sodium hydroxide and colloidal silica. A uniform crystalline membrane was removed and calcined. Membranes synthesized by this method were 20-230 .mu.m in thickness, had a XRD pattern characteristic of zeolite ZSM-5, and exhibited separation selectivities of 1.07 for O.sub.2 /N.sub.2, 1.62 for H.sub.2 /CO, and 17.2 for n-hexane/2,2-dimethylbutane. Diffusivities for these components ranged between 1.14.times.10.sup.-5 -2.63.times.10.sup.-4 cm.sup.2 /s.
A composite membrane having an ultrathin film of zeolite on a porous support of metal, inorganic, or polymeric material has been described by Suzuki (U.S. Pat. No. 4,699,892). This membrane acts as a molecular sieve by separating hydrocarbons of different chain lengths. For example, when an equimolar mixture of CH.sub.4, C.sub.2 H.sub.2 and C.sub.3 H.sub.8 was introduced into the feed side, gas chromatographic (GC) analysis showed that 73.5 mol % CH.sub.4, 26.0 mol % C.sub.2 H.sub.6, and 0.5 mol % C.sub.3 H.sub.8 were present on the separation side. No permeability values are provided for these membranes. The method of Suzuki was subsequently shown not to produce zeolite membranes of continuous coverage (European Patent Application Publication 481 660).
EP 481 660 describes the synthesis of a composite membrane comprised of crystals of zeo-type material on a porous support. The crystal growth was shown to be essentially continuous over the pores of the support and the zeolite material is crystallized and bonded directly to the support. This membrane was used for dehydration of aqueous alcohol solutions. Ceramic membranes offer many advantages over polymeric membranes, such as thermal, structural and chemical stabilities. However, unlike dense polymeric membranes, ceramic membranes are porous. It is difficult to produce ceramic membranes with pore sizes in the range of 10-30 .ANG., pore sizes of molecular dimensions useful for gas separation and pervaporation (Goldsmith (1988) supra). For this reason, ceramic membranes have not been used broadly in such processes.
Zeolite molecular sieves, having high porosities of about 30% of the total volume and definite pore sizes in the range of 3-12 .ANG., have been suggested to be good candidates for ceramic membranes. The difficulty remains on how to produce such zeolite membranes without non-zeolite micropores or defects. It would be advantageous to form a continuous zeolite layer on top of a ceramic layer with a resulting membrane of smaller pore size (e.g., 3-12 .ANG.) for use in a continuous separation process.
Jia et al. (1993) J. Membrane Sci. 82:15, discloses the synthesis of silicalite membranes on ceramic disks. SEM and XRD analysis demonstrated a continuous silicalite thin layer formed on the surface of the disks. Single gas permeation studies showed that the calcined membranes had high permeances for lightly adsorbed gas molecules. Evidence of shape-selectivity was observed, since n-butane permeated six times faster than iso-butane.